Top images: Canon 7D, Canon 70-200 f/4L IS lens, f/5.6 at 1/13 sec, ISO 100
When I photograph food the one thing I don’t want to do is blow-out/over-expose/clip any large areas of the frame. When you overexpose the highlights in a photograph you are pushing those pixels so that they are 100% pure white (255, 255, 255 on the RGB color scale). Even if you are photographing something that is actually white, you still don’t want to overexpose the whites because you will lose precious shadow detail in those areas. (One exception to this would be if you are isolating your subject on a white background in a studio environment.)
The top two photos in the above image were both photographed with the exact same settings, one with an over-exposed background, and one with a well-balanced exposure. The basic lighting setup, as you can see in the behind-the-scenes image, was window-light with the use of reflectors in the front of the food for fill-light (here’s a pulled-back image of my living room so you can see the size of the entire window).
Red Areas: The red you see in the images are the areas that are overexposed. For the first photo I had a good exposure on the food in the foreground but the background was way washed out. In the second “fixed” photo there is very little red, with the exception of a highlight along the rim of the bowl (which doesn’t really bother me). You can view the overexposed whites (and also the underexposed blacks) while editing your photos in pretty much any RAW editing software, and you can also enable a “highlight” alert on most SLRs that makes it really easy to spot the clipped highlights. (Check your camera manual for more information specific information on enabling the highlight alert on your brand/model of camera.)
Histogram: Notice the histogram towards the top of each of the photos. The tones on the first image are pushed all the way to the right, showing that the whites are “clipped”. In the second image the overall tones are mostly balanced, indicated by a full “mountain range” in the histogram.
The Fix: Since the too-bright-light was mostly coming in from the top-right table-top area of the photo, I dropped a piece of black foam core down behind it to cut out that “wash” of light hitting the table. The window is tall enough that the light still poured in from above the piece of black foam core, and I only lost a very small amount of light back-lighting the bowl of chips up front.